There is a need for expanding the scope of Ethernet area, including Ethernet, fast Ethernet, Gigabit Ethernet. Transferring Ethernet (defined by IEEE WG 802.3) over telecom physical channel is a simple and cheap technique to connect LANs, Internet/Intranet within a private and public network.
ITU-T G.707 describes the advantages offered by SDH and multiplexing method, and specifies a set of SDH bit rates, the general principles and frame structure of the network node interface (NNI), the overall frame size of 9 rows by N*270 columns, the section overhead(SOH) together with its byte allocation, arrangements for international interconnection of synchronous transport modules (STMs), the formats for multiplexing and mapping elements into the STM-N at the NNI.
The North America equivalent of SDH is SONET. SONET is the U.S. (ANSI) standard for synchronous data transmission on optical media. People ensure standards so that digital networks can interconnect internationally and that existing conventional transmission systems can take advantage of optical media through tributary attachments. SONET defines a base rate of 51.84 Mbps and a set of multiples of the base rate known as Optical Carrier Levels. The SONET is an octet-synchronous multiplex scheme that defines a family of standard rates and formats. Despite the name, it is not limited to optical links. Electrical specifications have been defined for single-mode fiber, multi-mode fiber, and CATY 75 ohm coaxial cable. The transmission rates are integral multiples of 51.840 Mbps, which may be used to carry T3/E3 bit-synchronous signals. It is also strong recommended to use G.703 E1/E3/E4/T1/E2/T4 interfaces as physical layer of IP-over-SDH/SONET. It is convenient user access way via LAN.
Both SDH and SONET provide standard for a number of line rates up to the maximum line rate of 9.953 gigabits per second (Gbps). Actual line rates approaching 20 gigabits per second are possible.
Existing data communication architecture of combination Ethernet with SDH/SONET uses PPP (point-to-point protocol) together with HDLC (high level data link control procedure). It is specified as RFC1619 in the IETF (Internet Engineering Task Force). However, when applying to the combination of Ethernet/Fast Ethernet/Gigabit Ethernet with SDH/SONET, RFC1619 presents following main defects:    (1) The whole application scheme has not been supported by a unified international standard, which results in the devices of different manufactures could not be unified when interconnected within a private or public network;    (2) for 2.5 Gbps rate or up, the overhead of hardware forwarding section is too heavy, especially for IP over WDM, since it is recommended to use LCP (link control protocol) and Magic Number both of which are very complicated, in RFC1619;    (3) in case the RFC1619 is used, the default value of the retransmission timer is 3 second in PPP, which is too slow for high speed link. For specific engineering applications, it is required to support all the rate range from 2M bit/sec to 1000M bit/sec (change about 4032 times), therefore the value of the retransmission timer should be determined based on the round-trip delay along the line. However, it is not defined in the RFC1619, resulting uncertainty when interconnecting equipment from different vendors;    (4) LCP contains 10 configuration packets, 16 events and 12 actions, and more than 130 protocol states, which is difficult and costly to implement for optical packet exchange between MII and SDH/SONET. For illustrating the above, Table 1 shows the list of Events and Actions using the conventional PPP over SONET/SDH standard on finite-state machine of LCP; and    (5) the padding field of PPP is almost not used in the case of IP over SDH/SONET, but it still kept now in RFC 2615. In addition, this padding field requires a function at the receiver side to distinguish between information field and padding field which would have been defined in RFC standard, and at the same time which in turn increases the processing overhead.
The most important feature of Ethernet over SDH/SONET (EOS) is that    (a) It will be used to both SONET/SDH telecom networks and Ethernet based datacom networks.            SONET/SDH equipment with Ethernet interface of long distance from site to site,        Ethernet switch with SONET/SDH interface.            (b) Add and drop 10/100M Ethernet when multi-chip is applied at the SONET/SDH terminal.    (c) It will be also used to linecard of Giga-router.
TABLE 1The list of Events and ActionsEventsActionsUp= lower layer is Uptlu= This-Layer-UpDown= lower layer is Downtld= This-Layer-DownOpen= administrative Opentls= This-Layer-StartedClose= administrative Closetlf= This-Layer-FinishedTO+= Timeout with counter > 0irc= Initialize-Restart-CountTO−= Timeout with counterzrc= Zero-Restart-Count  expiredRCR+= Receive-Configure-Requestscr= Send-Configure-Request  (Good)RCR−= Receive-Configure-Request  (Bad)RCA= Receive-Configure-Acksca= Send-Configure-AckRCN= Receive-Configure-Nak/Rejscn= Send-Configure-Nak/RejRTR= Receive-Terminate-Requeststr= Send-Terminate-RequestRTA= Receive-Terminate-Acksta= Send-Terminate-AckRUC= Receive-Unknown-Codescj= Send-Code-RejectRXJ+= Receive-Code-Reject  (permitted)  or Receive-Protocol-RejectRXJ−= Receive-Code-Reject  (catastrophic) or  Receive-Protocol-RejectRXR= Receive-Echo-Requestser= Send-Echo-Reply  or Receive-Echo-Reply  or Receive-Discard-Request
It could be seen from above analysis that the existing method to combine Ethernet with SDH/SONET is complex, difficult and expensive to implement, slow, unstable, and not suit for high speed data transmission, especially for Gigabit rate application.